How do you get at least four hits from machine gun upper?(Super Turbo)

This move is extremely difficult to get perfectly but does an ungodly amount of damage if you can get 4-5 hits off of it. However I can only seem to get 3 when I mash all of the punch buttons at the same time and then stop for the knockdown hit.

You have a stick?
Either piano key like crazy or use a different type of button tapping (putting all your fingers into a point and mashing p. Old 3s Chun-li trick, but applied to punches.)